Catheterization of the cardiovascular system may be performed for any of a number of diagnostic and therapeutic reasons. For example, catheterization may be used to implant electrodes of a pacemaker into the heart of a subject needing a pacemaker or for balloon angioplasty where a balloon is inflated in an artery of a subject thereby widening restricted blood vessels. Catheterization is also useful in relation to certain surgical procedures for the repair of damaged blood vessels. Additionally a host of other medical procedures require incorporeal placement of other invasive devices, and precise knowledge of device position within the body.
One difficulty in performing medical procedures involving catheterization lies in guiding the catheter through blood vessels of the cardiovascular system. As the catheter moves through the blood vessel, the catheter is prone to taking "wrong turns" into secondary blood vessels leading away from the target of the intervention.
To solve the problem of locating a probe such as a catheter within a blood vessel, fluoroscopy has been relied upon in conjunction with the use of an iodine contrast medium introduced into the blood of the subject. Fluoroscopy, in fact, has been used since 1950 in retrograde aortic root and left and right ventricle catheterization and angioplasty for accurate catheter guidance.
Other methods of determining a location of a probe inside blood vessels of the cardiovascular system have included ultrasonic imaging techniques. Using ultrasound, sound waves are transmitted from an ultrasonic transducer through the soft tissue of the subject. Upon striking the probe, the sound waves are reflected back to the ultrasonic transducer where the reflected sound waves are detected and an image of the reflecting object is displayed on a monitor.
Two dimensional ultrasonic images are displayed on the monitor in a pie-shape format. The apex of the pie represents the relative position of the ultrasonic transducer and ultrasonically reflecting objects are displayed radially from the apex with the imaging field. Relative positioning of the reflecting object within the display is determined by the length of time required for a reflected sound wave to return to the transducer and from the relative angle of the sound wave with respect to the transducer.
While ultrasonic imaging can provide important information relative to probe position, it is limited in at least two aspects. First, ultrasound does not easily pass through bone. More to the point, bone reflects ultrasound sufficiently to mask, and thereby render invisible, important structures lying behind the bone.
Secondly, an ultrasound image (e.g., B-mode) is a two-dimensional display, providing information with respect to a small area of a two-dimensional plane passing through the body. Where a catheter passes orthogonally through the plane, the ultrasonic image may display a small circular shape, representative of the diameter of the probe. By moving the ultrasonic transducer, an operator may be able to obtain more information (i.e., by aligning the plane of the ultrasound transducer with a portion of the length of the probe). However, if the probe tube is deflected in more than two dimensions (a highly likely event in most cases) the operator will not be able to identify an insertion end of the probe much less the orientation of the insertion end with respect to the catheterized subject. In addition, some cardiac structures, such as valvular apparatus and aortic wall, especially if highly echogenic, can be mistaken as the catheter tip.
One advance in obtaining the position of the insertion end of the catheter has been provided by U.S. Pat. No. 4,706,681 to Breyer, et al. In Breyer et al. a piezoelectric transducer was placed at the insertion end of the catheter for the transmission of a marker signal to the ultrasonic transducer.
While Breyer et al. is effective in locating the insertion end of the catheter, Breyer et al. still is relatively ineffective in providing information as to the direction in which the catheter end is pointing. When the end of the catheter has doubled over or has entered an incorrect blood vessel, it is still difficult, if not impossible, to identify such conditions. Because of the importance of catheterizations to heart patients, a need exists for an apparatus and method of determining an orientation of a probe within a biotic structure.